Circuit board testing device for uneven circuit boards

ABSTRACT

A circuit board tester and method that precisely aligns the probe plate and circuit board is disclosed. With a circuit board and probe plate mounting within a housing having a top and bottom, hinged together, at closure there may be slight misalignments of the two. By making one of the two plates floating, or laterally slideable with respect to each other, it is possible to make final alignment at closure. One of the two plates can be provided with a pin and the other with a pin receiving alignment block. With the lateral slideability, the pin and block can insure proper probe alignment. Additional systems for correcting misaligned pins or blocks are also disclosed.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed to test equipment, and more particularly totest equipment for circuit boards.

2. Description of the Related Art

Circuit board testers are used for testing a variety of circuit boardsor similar devices to assure that the circuit boards operate asintended. In at least one type of circuit board tester, such as AgilentModel No. 3070, Series 3, a separate device, referred to as a fixture,is used to position the circuit board such that a plurality ofelectrically conductive probes (which are part of, or coupled to, thetester) contact predetermined components or positions of the circuitboard. The particular components or positions that are contacted by thetest or probes depend on the tests that are desired. When the probes arein contact with the desired locations on the circuit board, electricalsignals with predetermined parameters (e.g., predetermined magnitudes orpatterns of current, voltage frequency, phase and the like) are appliedby the tester, typically under control of a computer, to certain of theprobes. Some or all of the probes are used to measure the performance orresponse of the circuit board (i.e., to measure electrical parameters atsome or all of the probes contacting the circuit board). In this way, itis possible to rapidly perform a number of tests or measurementscharacterizing the performance of the circuit board while simulating theconditions the circuit board would have, or could have, during actualuse. Although it is possible to use these types of tests (and testingdevices) for a variety of possible purposes (such as “spot checking”selected circuit boards at a production facility, testing circuit boardswhich may be malfunctioning, testing prototype circuit boards as part ofa design program and the like), in at least some applications, circuitboard testing is used to provide quality assurance on all orsubstantially all products of a given type or class which are producedby a company. Even with the relatively rapid test procedures which canbe achieved by circuit testing, it is not unusual for desired testing ofeach circuit board to require on the order of 30 seconds to 90 secondsor more.

Because, in at least some applications, circuit board testing isperformed on substantially all devices on a production line orproduction facility, reliability of testing can be especially importantsince delay or failure at a testing station can delay or interrupt theoverall production in a production line or facility and cause expensiveboards to be rejected. One cause of rejection or test failure is unevenor warped circuit boards.

Indeed, warped boards are quit common. The larger the board, the greaterthe likelihood that the board will not be sufficiently flat to conductan accurate test. One reason why warped boards can fail a test, is thatthe warpage causes the probe to miss their target.

It as been assumed, that the solution to warped and uneven boards is toapply sufficient pressure on the board to even it out during test. Thismay work for minor warpage, but warpage of some degree will eitherdamage the testing device by requiring so much pressure to make adequateprobe contact or destroy the board. The board can be destroyed in thisflattening process by snapping off surface mounted, galling the board,cracking ICs and other components, breaking leads, breaking traces or byother means which render the board unusable or untestable.

Accordingly, it would be useful to provide a fixture, useable inconnection with in-circuit testers, which provides desired speed ofpositioning the circuit board or other unit under test (UUT) and whichachieves a relatively high degree of reliability, and can test unevenboards without damage.

BRIEF SUMMARY

This summary is supplied to assist the reader in understanding theremaining disclosure and does not define of the scope of the invention.

The present disclosure relates to a system and method for insuringalignment of a probe plate with a unit under test (UUT) by bringingphysical and electrical contacts into reliable and repeatable alignmenteven where the board (UUT) is not flat or warped. The disclosed featurecan be added to existing circuit testing devices of almost any kind, sothe disclosed testing device is only illustrative on one exemplaryenvironment.

The following are some of the features of the disclosure.

There is disclosed a circuit tester for testing circuit boards having ahousing; a first board mounting plate for receiving circuit board unitunder test (UUT); a retaining plate associated with the second part ofsaid housing, said retaining plate having a plurality of retainingfingers oriented toward the UUT, said retaining fingers being generallyorthogonally slideable and lockable in said plate, said UUT andretaining plate being moveable toward each other, a locking plate forengaging said fingers and locking its orthogonal position relative tothe retaining plate, so that when said UUT and retaining plate arebrought toward each other during circuit test, said retaining fingersare allowed to engage the UUT at differing heights according to theflatness of the board, and wherein said fingers are then locked againstthe UUT by engaging said locking plate.

Also disclosed is a self adapting tester capable of adapting to test fortesting a circuit board/unit under test (UUT) regardless of the board isplanar, having a bed for receiving a UUT a push finger plate including aplurality of spaced apart apertures, a plurality of push fingers sizedto be sildeably received with in said apertures, a locking plategenerally coplanar with said push finger plate said locking plateincluding a plurality of apertures generally collinearly aligned withsaid apertures in said push finger plate, a drive for moving one of saidplates relative the other so that said plates have a first positionwhere the apertures are generally collinearly aligned and secondposition where the apertures are partially out of alignment, so that thepush fingers are capable of sliding generally orthogonally thru theplates when they are in the first position and are locked from slidingwhen the plates are in the second position, so that pressure isgenerally applied equally across the UUT by the push fingers when theyare allowed to contact the UUT in the first position and locked in thesecond position.

Also disclosed is a tester wherein the apertures, when collinearlyaligned are equal to or greater than the diameter of the fingers andwhen not collinearly aligned are less than the finger diameter.

Also disclosed is a tester wherein the apertures on one of the platesare non-round.

Also disclosed is a tester wherein the locking plate moves laterally andthe finger plate is laterally static.

Also disclosed is a tester, wherein the fingers include a barrel portionand wherein at least a portion of the barrel passes through theapertures and where that portion includes an area of increased friction.

Also disclosed is a tester, wherein said area of increased frictionincludes a knurled portion.

Also disclosed is a tester, wherein said area of increased frictionincludes a high friction coating.

Also disclosed is a tester, wherein the push fingers include a centralcore pin and a collet surrounding said pin and wherein said core pin islocked by compression of the collet.

Also disclosed is a tester, where said fingers include an actuator whichextends and retracts the length of the finger in response to forceapplied.

Also disclosed is a tester, wherein the force is selected from the groupof: pneumatic, hydraulic, electric or electromagnetic forces.

Also disclosed is a tester, wherein said fingers further include a stopat their end most proximate finger plate, said stop configured toprevent the pin from passing completely thru the plate.

Also disclosed is a tester for testcircuit boards having a housing; afirst board mounting plate for receiving circuit board/unit under test(UUT); a retaining plate associated with the second part of saidhousing, said retaining plate having a plurality of retaining fingersoriented toward the UUT, said retaining fingers being generallyorthogonally slideable and lockable in said plate, said UUT andretaining plate being moveable toward each other, a locking plate forengaging said fingers and locking its orthogonal position relative tothe retaining plate, so that when said UUT and retaining plate arebrought toward each other during circuit test, said retaining fingersare allowed to engage the UUT at differing heights according to theflatness of the board, and wherein said fingers are then locked againstthe UUT by engaging said locking plate.

Also disclosed is a tester, for testing circuit boards having a housing;a first board mounting plate for receiving circuit board/unit under test(UUT); a retaining plate associated with the second part of saidhousing, said retaining plate having a plurality of retaining fingersoriented toward the UUT, said retaining fingers being generallyorthogonally disposed in said plate and having an actuator for extendingand retracting their length, said UUT and retaining plate being moveabletoward each other, so that when said UUT and retaining plate are broughttoward each other during circuit test, said retaining fingers areactuated to contact said UUT at differing heights according to theflatness of the board, and wherein said fingers are then locked againstthe UUT by engaging locking the actuator against further length changeof the fingers.

There is also disclosed a method of circuit testing of a circuitboard/united under test (UUT) in a circuit tester having a UUT moveableretaining plate having a plurality of retaining fingers slideablyextending toward the UUT comprising the steps of:

-   -   a. bringing the fingers of the retaining plate in contact with        the UUT;    -   b. allowing the fingers to slide generally orthogonally in the        retaining plate until they contact the UUT;    -   c. locking the fingers from sliding relative to the retaining        plate;    -   d. conducting electrical tests of the UUT.

Also disclosed is a method of precisely applying pressure on a UUT witha probe plate having a plurality of vertically actuateable lockingpressure fingers, comprising the steps of:

mounting the UUT on either side of a hingeable housing;

mounting a probe plate with said pressure fingers on the other side ofthe hingeable housing;

providing acutatable engagement of either the probe plate or UUT withrespect to its side of the housing to bring the probe plate and fingersin contact;

adjusting the length of the fingers so that each engages the UUTregardless of the surface height of the UUT at each point of contact;

locking the position of the fingers to prevent further movement thereof;moving the UUT and probe plate into closer contact according to apredetermined desired test position. The fingers may apply differentpressure in different points on the board.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a perspective view of a circuit board tester from U.S. Pat.No. 8,004,300 in open position as an environmental view;

FIG. 2 illustrates a schematic side view of a circuit board tester inrest, first stage;

FIG. 3 illustrates a schematic side view of a circuit board tester in asecond stage where the fingers first engage the UUT;

FIG. 4 illustrates a schematic side view of a circuit board tester in athird, test stage position with fingers locked;

FIG. 5 is a fragmentary enlarged view of a portion of FIG. 4 showingfingers in a locked state;

FIG. 6 illustrates an alternative embodiment with a collet-style lockingfinger;

FIG. 7 is a close up view of the collet-style locking finger;

FIG. 8 is a schematic top view of retention and locking plate holealignment;

FIG. 9 is a schematic top view of retention and locking plate hole wherethe locking plate hole is diamond shape;

FIG. 10 is a schematic top view of retention and locking plate holewhere the locking plate hole is oval or has taper ends;

FIG. 11 is a perspective view of a pin/finger assembly;

FIG. 12 is a perspective view of an alternate pin/finger assembly;

FIG. 13 is a perspective view of a collet in useable with a centralshaft as an alternative to the above pin assembly;

FIG. 14 is a side plan view of FIG. 3;

FIG. 15 is a side perspective view of FIG. 3;

FIG. 16 is an exploded perspective view from above of FIG. 15;

FIG. 17 is an exploded perspective view from below of FIG. 15;

FIG. 18 is a view of an actuateable push finger.

DETAILED DESCRIPTION

In a manufacturing environment for circuit boards, a final test willoften be an electrical test, to ensure that each circuit board performsas required. Such tests are well-known in the industry, and may beperformed by commercially available testers, such as Agilent Model 3070.Many boards are not flat. They may be warped or they may intentionallynon-planar to fit enclosure requirements. This disclosure providessolutions which make testing of non-planar boards possible.

A detailed view of the mechanical configuration is shown in FIG. 1, aprior art system described in U.S. Pat. No. 8,004,300 incorporated byreference and shows a testing environment. The UUT (unit under test) isshown as a circuit board 16 and is removably and rigidly attached to,and optionally, spaced apart from, a support plate or mounting plate 18with or without spacer elements. In this configuration, the electricalcontacts on the UUT 16 that are to be tested face upward, and areaccessible to various probes from above or below plate 18. There mayalso be probes from underneath the UUT 16. In this case, probes from thetop are illustrated. Note that the circuit board and fixture on which itis mounted are considered one for the purpose of this application,though they are likely to be separate components. We refer to thecircuit board whether or not it includes a fixture. The probes may applyand measure voltages or currents at various locations on the UUT, andare controlled mechanically and electrically by the tester. A computer,not shown, may control the tester and may record data from the tests.

The testing system 10 is shown as a box with a top 12 with handle 20holding a probe plate 22. Probe plate 22 is configured to be freelymoveable in lateral directions and optionally to a limited degree alonga vertical axis which passes orthogonally thru the UUT 16 which sites ona “bed” in the bottom portion of the box, and contacted by the probeplate 22 when the box is closed and actuated as will be explained. Onthe probe plate are of course electrical probes, but also a plurality ofpins or retaining or push fingers which engage the UUT to hold it inplace during the test. In prior art systems, it is these pins/fingerswhich can damage a warped board because they are fixed in length andapply pressure to the UUT unevenly when the UUT is uneven. If forexample the UUT has a high portion, the fingers will effectively try tocrush the board into a flat plane. This is highly undesirable and candestroy or even pierce the board or snap off components.

The bottom of the system 10 includes a support plate 18 which supportsthe UUT 16 preferably rigidly in place on plate 18. There are severalways to accomplish this rigid connection. The preferred way is by posts30 which surround the UUT and create a rigid perimeter. The posts mayengage notches in the UUT board, but they may also simply be placedaround the periphery to inhibit movement. In some circumstances, the UUTmay have existing apertures which allow it to be affixed to the supportplate.

Hinges 25 allow the top 12 and support 18 to move relative to eachother. Pneumatic cylinders 24, regulate the movement of the top andplate.

In may cases the circuit board (UUT) is not perfectly flat. Aspreviously mentioned unevenness or warpage creates serious problems withreliable contacting of probes on the UUT, which are compounded in dualstage for bi-level systems which have short and long probes forin-circuit and functional tests. Warped boards create misalignment andincorrect pressure application in the Z-axis. Other methods devised formisalignment the X-Y axes do not address Z-axis misalignment andpressure issues.

FIG. 2 illustrate schematically a testing device modified to include thenew disclosure and concepts. Hinge 24 provides a pivot for the topportion 12 to engage the bottom portion 13. A latch system 42 is shownbut alternative latches are suitable.

FIG. 2 shows the system in a “rest” state, ie not in test mode. Probeplate 24 (also known as a jumper plate) contains a plurality of holes 44through which push down pins/retainer/push fingers 50 are freelyslideable. This means that the holes are at least slightly larger thanthe diameter of the pins 50.

Pins/Fingers 50 are cylindrical rods of a rigid material like steel orplastic. A close up view is found in FIG. 5. Resilient materials mayalso be used. The preferred embodiment has the rigid material covered bya more compressible or higher friction sleeve 52. One such material is acompressible plastic or a knurling on the rigid surface to create a highfriction surface. These embodiments help in creating frictionalengagement when the pins/fingers are locked against sliding movement.

At the distal end of each finder is preferably a tapered section 54 witha flat end. The taper reduces the contact area which is advantageous forcrowded boards. The taper and be much more pointed but should preferablynot have a sharp end. Stop or end cap 58 prevents the finger fromsliding entirely thru the board.

At the proximal end, the pin includes a retainer which may be any numberof elements which enlarge the end to prevent the pin from falling out. Acirclip retainer is suitable as would be a bulbus end.

Relatively adjacent the jumper plate 24 is a second moveable laterallysliding plate 60 and a fixed retainer plate 62. The sliding plate 60 hasa plurality of holes similar spaced in the same pattern as holes 44 inplate 24 but of greater diameter. This increased diameter allows theplate 60 to slide laterally in parallel with plate 24 but in its neutralposition (shown in FIG. 2.) not engage the pins/fingers 50. The platesare preferably generally coplanar and then moved relative to each other,ie plate 60 (or plate 24) is moved laterally by a drive preferablyhaving remote actuation (electrical/mechanical/pneumatic) or manualdrive lever so that the holes in one plate and the holes in the otherimpinge/pinch upon the pin/finger preventing their movement. If theplates are not coplanar, the apertures need to be larger to accommodatethe pins in their aligned position.

FIG. 3 shows the system in the engagement (first) stage where the pushfingers 50 engage various points on the UUT 16 after having moved towardthe UUT along the Z-axis. The unevenness of the board can be caused bythe board itself or by components thereon. The board is shown as flatbut the components supply an example of unevenness. (Most often thefingers are placed between components but component contact cannotalways be provided on densely populated boards. Warpage is not easilyshown in the drawings but can be presumed to exist on many boards.Notice that whether by warpage or components, the fingers adjust to theactual height of that portion of the board. No attempt is made via thefingers to flatten the board. They simply conform to the curvature ofthe board. The fingers are free to rise or fall under the force ofgravity to their equilibrium position. FIGS. 14, 15, 16, and 17 provideadditional views to understand the entire system in various views.

FIGS. 4-5 show the UUT in test stage. The probe/jumper plate 24 is intest position (probes are not shown) but now the fingers 50 are lockedinto place. Sliding plate 60 has moved relative to jumper plate 24 sothe combined diameter/area of the holes is less than or equal to theouter peripheral diameter of the shaft of the pins 50. In essence, thepins are trapped/locked between the opposing lateral forces of the twoplates 24-60, so that they can no longer move longitudinally, iegenerally orthogonally to the holes. The UUT is thus engaged in itsposition, accommodating for unevenness and warpage. Components are notoverstressed by an attempt to reshape the board.

FIGS. 6 and 7 illustrate an alternative locking system. Instead offriction engagement with the shaft of the pin by the edges of the holes60 and 44, it is possible to utilize a collet 76 in the hole and havethe other plate with holes compress the collet around a core pin (notshown) with vertical instead of lateral movement to compress the collet.FIG. 6 shows plate 24 with a recessed depression 78 sized to receive thecompressible part of the collet 76. See also FIG. 13 which shows thecollet in close up perspective with a compressible portion 76 a and aplurality a slots 76 b spaced around to allow for compression. Plate 60is slide downward to drive the collet 76 in the recess 78. Thiscompresses the collet around the finger shaft 50 locking it againstsliding in the collet and thus maintaining its position relative to theUUT. The recess 78 is shown as V-shaped but may be other shapesconfigured to cause the collet to compress, such as a bulbous shape or acylindrical recess.

FIG. 8 is a schematic top view of the two plates 24 and 60 shown withtheir respective holes 44 and 64 intentionally misaligned to show thereduction in the composite hole diameter which locks the pin 50. FIGS. 9and 10 show alternative embodiments where one of the plates preferably,the upper plate 60 has a non-circular opening that tapers to one or bothends. Ovals, diamonds, triangles, etc, meet these characteristics. Bysliding the plate with tapered apertures toward the aperture, the pin orcollet is thereby locked. The locking/pinching action can be achieve solong as the resultant diameter of the collinearly aligned apertures isless than the diameter of the finger when the apertures are move out ofcollinear alignment. One solution is that the apertures in on plate arenot round or have sides with a lesser cross section or diameter.

FIG. 11 is a close up view of a preferred pin 50 with a tapered tip, ashaft 51 and a slightly compressible overlay 53 which in this case is aplastic sheath. FIG. 12 is similar (with tapered end removed) but has aknurled surface 51 on it shaft. This may be useful in holding the sheath53 in place, or the sheath may be dispensed with as the knurled body issufficiently high friction to engage the holes 44 and 60 in a lockedstate. The shaft should in the preferred embodiment be de-burred or evensmooth to prevent hanging on a hole and not traveling freely in responseto gravity. To remove the risk of hanging on pins, the board can bevibrated during movement or the pins can be driven down onto the UUTindependently of the plate's movement. This could be down by an air jet,a separate backing board which has a resilient surfaces (such as foam)and which drives the pins downward or by mechanical, electrical, orpneumatic systems.

In certain conditions, it may be desirable to apply greater pressure onthe board/uut than has been described. This can be accomplished by themethod as described with locking of the pins/fingers, but then after thepins are locked in position, driving the pins (and associated probesslightly further toward the UUT. This will increase the overall pressureon the UUT but it will still be even pressure, ie take into account theunevenness of the board. In such a way, the pressure can be uniformlyincreased without risk of damage to the UUT.

Alternative to the gravity fed fingers 50 are contemplated by thisinvention. FIG. 18 is a view of an actuateable push finger. Finger 50can include a drive cylinder (electrical, solenoid, hydraulic,pneumatic, etc.) which has a first fixed portion which is attached toone of the plates, (62 for example) and a distal portion which slidesrelative to the fixed portion, in response to force, such as fluid,magnet force, etc. Hydraulic/solenoid cylinders are know in the art inother fields such a fuel injectors (for example hydraulic cylinders fromCustom Cylinders Inc. 700 Industrial Dr, Cary, II 60013 or solenoidssuch as in US patent publication US 2005/0103882). So instead of usinggravity to allow the fingers to drop into place, a force drives anextension of a pin onto the UUT. The advantage of a ‘force driven” or“powered” finger array is that it is position independent. Therefore, itis possible to have the fingers project upward (such as from the bottom)or sideways. This may also eliminate finger “hanging” and likewiseeliminates the need to a locking plate if desired, because the forceapplied can be maintained or released remotely and each finger could becontrolled independently by a pressure lock value or electronic control.Different forces could be applied to each finger independently sogreater or less forces could be applied to fingers touching the basecircuit board rather than delicate components.

Thus, a tester built with fingers which are actuatable (ie have anactuator, perhaps internal, such as a solenoid or pneumatic/hydrauliccylinder) could have a housing; a first board mounting plate forreceiving circuit board/unit under test (UUT); a retaining plateassociated with the second part of said housing, said retaining platehaving a plurality of retaining fingers oriented toward the UUT, saidretaining fingers being generally orthogonally disposed in said plateand having an actuator for extending and retracting their length, saidUUT and retaining plate being moveable toward each other, so that whensaid UUT and retaining plate are brought toward each other duringcircuit test, said retaining fingers are actuated to contact said UUT atdiffering heights according to the flatness of the board, and whereinsaid fingers are then locked against the UUT by engaging locking theactuator against further length change of the fingers.

Likewise a method of testing a UUT with an acutatable finger willactuate the fingers to contact the UUT at varying contact heights andthen the actuator will be locked so that the finger length is then fixedduring test. It is likewise possible to also have the actuator increasethe pressure after contact to a predetermined level, or have certainfingers have a greater level of pressure, such as those which extend thelongest or past a predetermined length, which might indicate they aresituated on the board not a more fragile circuit component. It is alsopossible to use this actuatable finger concept to apply selectivepressure on the board to flatten it, though that may be desirable onlyin certain circumstances.

The description of the invention and its applications as set forthherein is illustrative and is not intended to limit the scope of theinvention. Variations and modifications of the embodiments disclosedherein are possible, and practical alternatives to and equivalents ofthe various elements of the embodiments would be understood to those ofordinary skill in the art upon study of this patent document.

These and other variations and modifications of the embodimentsdisclosed herein may be made without departing from the scope and spiritof the invention.

The invention claimed is:
 1. A self adapting tester capable of adaptingto test for testing a circuit board/unit under test (UUT) regardless ofthe board is planar, comprising: a. a bed for receiving a UUT; b. a pushfinger plate including a plurality of spaced apart apertures; c. aplurality of push fingers sized to be sildeably received within saidapertures; d. a locking plate generally coplanar with said push fingerplate said locking plate including a plurality of apertures generallycollinearly aligned with said apertures in said push finger plate; e. adrive for moving one of said plates relative the other so that saidplates have a first position where the apertures are generallycollinearly aligned and second position where the apertures arepartially out of alignment, so that the push fingers are capable ofsliding generally orthogonally thru the plates when they are in thefirst position and are locked from sliding when the plates are in thesecond position so that pressure is generally applied equally across theUUT by the push fingers when they are allowed to contact the UUT in thefirst position and locked in the second position.
 2. The tester of claim1 wherein the apertures, when collinearly aligned are equal to orgreater than the diameter of the fingers and when not collinearlyaligned are less than the finger diameter.
 3. The tester of claim 1wherein the apertures on one of the plates are non-round.
 4. The testerof claim 1 wherein the locking plate moves laterally and the fingerplate is laterally static.
 5. The tester of claim 1 wherein the fingersinclude a barrel portion and wherein at least a portion of the barrelpasses through the apertures and where that portion includes an area ofincreased friction.
 6. The tester of claim 5 wherein said area ofincreased friction includes a knurled portion.
 7. The tester of claim 5wherein said area of increased friction includes a high frictioncoating.
 8. The tester of claim 1 wherein said push fingers include acentral core pin and a collet surrounding said pin and wherein said corepin is locked by compression of the collet.
 9. The tester of claim 1where said fingers include an actuator which extends and retracts thelength of the finger in response to force applied.
 10. The tester ofclaim 9 wherein the force is selected from the group of: pneumatic,hydraulic, electric or electromagnetic forces.
 11. The tester of claim 1wherein said fingers further include a stop at their end most proximatefinger plate, said stop configured to prevent the pin from passingcompletely thru the plate.
 12. A circuit tester for testing circuitboards having a housing; a first board mounting plate for receivingcircuit board/unit under test (UUT); a retaining plate associated withthe second part of said housing, said retaining plate having a pluralityof retaining fingers oriented toward the UUT, said retaining fingersbeing generally orthogonally slideable and lockable in said plate, saidUUT and retaining plate being moveable toward each other, a lockingplate for engaging said fingers and locking its orthogonal positionrelative to the retaining plate, so that when said UUT and retainingplate are brought toward each other during circuit test, said retainingfingers are allowed to engage the UUT at differing heights according tothe flatness of the board, and wherein said fingers are then lockedagainst the UUT by engaging said locking plate fingers and furtherincluding a mechanism for moving one of said plates relative the otherso that said plates have a first unlocked position and second lockedposition where the retaining fingers are locked at different heights byengagement with at least one plate so that said fingers are locked fromsliding when the plates are in the second position.
 13. A circuit testerfor testing circuit boards having a housing; a first board mountingplate for receiving circuit board/unit under test (UUT); a retainingplate associated with the second part of said housing, said retainingplate having a plurality of retaining fingers oriented toward the UUT,said retaining fingers being generally orthogonally disposed in saidplate and having an actuator for extending and retracting their length,said UUT and retaining plate being moveable toward each other, so thatwhen said UUT and retaining plate are brought toward each other duringcircuit test, said retaining fingers are actuated to contact said UUT atdiffering heights according to the flatness of the board, and whereinsaid fingers are then locked against the UUT by engaging locking theactuator against further length change of the fingers and furtherincluding a mechanism for moving said plate so that the retainingfingers are locked from sliding generally orthogonally thru the plateswhen they are in the locked position.